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Dosimetric Impact Associated with the New 25-Source GammaPodTM Design

P Sabouri1*, Y Niu2 , C Yu2 , E Nichols1 , M Guerrero1 , B Yi1 , S Becker1 , (1) University of Maryland School of Medicine, Baltimore, MD, (2) Xcision Medical Systems, Columbia, MD


(Saturday, 3/30/2019)  

Room: Exhibit Hall

Purpose: The GammaPod (GPOD) is a novel prone-breast stereotactic radiotherapy device. Highly conformal plans are delivered using conical non-coplanar arcs from Co-60 sources with 18 or 25 mm collimation. The original GPOD design used 36 sources located between 18 to 53Ëš below the horizontal. The new design uses a more lateral placement of 25 sources (25S) spanning from 18 to 42Ëš below the horizontal. We assess the dosimetric impact, delivery accuracy and feasibility of the 25S-GPOD.

Methods: 25S-GPOD plans were generated using the planning CT scans and contours of 13 patients who received an 8 Gy (RX=8Gy) to 95% of the PTV boost on the 36S-GPOD. Delivery feasibility and accuracy was evaluated using patient-specific QA with a point-dose ion chamber measurement in an acrylic phantom, and relative gamma analysis with the SRS-MAPCHECK device (Sun Nuclear). The statistical significance of the reduction in OAR max-dose and differences in PTV coverage (D95%, D2%, D95%) were determined using a paired sample t-test.

Results: On average, the maximum heart dose was reduced by 3% (p-value=0.0047). Similarly, the average maximum lung dose was reduced by 2.8% (p-value=7e-6). The 25S-GPOD plans used the 15-mm collimator more frequently; while PTV coverage remained unchanged, use of the smaller collimator resulted in more conformal plans and an 18% average (p-value=0.009) reduction was observed in the conformity index. However, a systematic 1% increase (p-value=6e-5) between the 25S-GPOD TPS and measured point-dose was observed and is associated with more frequent use of the 15-mm collimator.

Conclusion: While PTV coverage remained unchanged, better heart and lung sparing was achieved using the 25S-GPOD design. This new design also exhibits a more frequent use of the 15-mm collimator, which resulted in more conformal plans at the cost of a slight increase between the measured and planned dose difference.

Funding Support, Disclosures, and Conflict of Interest: Ying Niu and Cedric Yu are affiliated with Xcision Medical Systems.

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